The present invention relates to neural stimulation systems and, more particularly, to an output control system that automatically maintains the output of the stimulation system at a comfortable and efficacious level.
The present invention may be used in various stimulation therapies in which a neurostimulator is used to stimulate neural tissue. One example where the present invention may be employed is with stimulation of peripheral nerves, e.g., the nerves in the arms, legs, fingers, which nerves are distant from the spinal cord. The present invention may also be used in stimulation of spinal cord nerves.
Spinal cord stimulation (SCS) systems, treat chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array placed epidurally near a patient's spine. SCS is a well-accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an Implantable Pulse Generator (IPG) coupled to an array of electrodes at or near the distal end of an electrode lead. An electrode lead extension may also be used, if needed. The IPG generates electrical pulses that are delivered to neural tissue, e.g., the dorsal column fibers within the spinal cord, through the electrodes of the electrode array. In an SCS system, for example, the electrodes are implanted proximal to the dura mater of the spinal cord. Individual electrode contacts (the “electrodes”) may be arranged in a desired pattern and spacing in order to create an electrode array. Individual wires, or electrode leads, connect with each electrode in the array. The electrode leads exit the spinal cord and attach to the IPG, either directly, or through one or more electrode lead extensions. The electrode lead extension, in turn, when used, is typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.
The electrical pulses generated by the SCS stimulation system, or other neural system, are also referred to as “stimulation pulses”. In an SCS system, the stimulation pulses typically have the effect of producing a tingling sensation, also known as a paresthesia. The paresthesia helps block the chronic pain felt by the patient. The amplitude or magnitude of the stimulation pulses affects the intensity of the paresthesia felt by the patient. In general, it is desirable to have the amplitude of stimulation comfortably set to a level which produces paresthesia to block pain but not above a level that may actually result in pain apart from the native pain. Moreover, the stimulus amplitude should be set to a stimulus level lower than that which can recruit reflex motor nerves that can cause involuntary muscle contractions.
SCS and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467, teaches an electrode implant for neuro-stimulation of the spinal cord. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted, and which is controlled by an external controller.
Representative techniques known in the art for providing for the automatic adjustment of stimulation parameters of an implantable stimulator are disclosed, e.g., in U.S. Pat. Nos. 5,895,416; 5,735,887; and 4,735,204.
Patients having an SCS system have heretofore had to manually adjust the amplitude of the stimulation pulses produced by their SCS system in order to maintain the paresthesia at a comfortable level. This is necessary for a number of reasons. For example, postural changes, lead array movement (acute and/or chronic), and scar tissue maturation, all affect the intensity of the paresthesia felt by the patient. Because of these changes, i.e., because of postural changes, lead array movement, and scar tissue maturation, as well as other changes that may occur in the patient, the paresthesia can be lost, or can be converted to painful over-stimulation, thereby forcing the patient to manually adjust the output. There is a need for a method or system that would eliminate, or at least mitigate, the need to perform such manual adjustments. Such method or system would be of great benefit to the patient.